//三步问题
//测试链接：https://leetcode.cn/problems/three-steps-problem-lcci/
public class WaysToStep {
    public int waysToStep(int n) {
        if(n==1) return 1;
        if(n==2) return 2;
        if(n==3) return 4;

        long[] dp = new long[n+1];
        dp[1] = 1; dp[2] = 2; dp[3] = 4;
        for(int i = 4; i <= n; i++) {
            dp[i] = (dp[i-1]  + dp[i-2]  + dp[i-3] ) % 1000000007 ;
        }

        return (int)dp[n];
    }
}
